1. Field of the Invention
This invention relates to a liquid crystal display apparatus for displaying a picture on a liquid crystal display panel employing a thin film transistor (TFT), and more particularly to a liquid crystal display apparatus having a residual image eliminating function so that it is capable of displaying a fine picture without a residual image.
2. Description of the Prior Art
Recently, there has been accelerating the development of a flat display device of the so-called active matrix driven type, for example, a liquid crystal display device employing TFTs. Since such a liquid crystal display device can be miniaturized compared with the Brown tube or cathode ray tube, it is commercially available in the market as a display device such as a portable television, lap-top type personal computer, or the like. Further, this liquid crystal display device reverses the polarity of a voltage applied to a liquid crystal cell every frame in order to reduce the driving voltage of the liquid crystal display panel.
In the liquid crystal display device employing TFTs, however, the voltage applied to the liquid crystal changes symmetrically due to the parasitic capacitance of the TFTs. This brings about a deterioration of the picture displayed on the liquid crystal causing the appearance of a residual image on the liquid crystal display panel. In order to overcome such picture deterioration, the conventional liquid crystal display device modulates the common voltage applied to the liquid crystal cell. The conventional device, however, still displays a residual image when a different picture is displayed after the same picture had been displayed for a long time. This results from the direct current (DC) component of the applied voltage being accumulated in the liquid crystal cell by a certain amount according to the progress of the frame in the case where the same picture has been displayed on the liquid crystal display panel for a long time. This phenomenon will be explained in more detail with reference to the drawings below.
Referring to FIG. 1, there is shown a picture element or pixel cell of a liquid crystal display panel which comprises a TFT 10 having a gate connected to a scanning line 11 and a source connected to a data line 13, a liquid crystal cell 12 connected between a drain of the TFT 10 and a common voltage source V.sub.COM, and an auxiliary capacitor 14. The TFT 10 is selectively turned on by a scanning control signal Vg in a pulse form on scanning line 11 to connect the data line 13 to the liquid crystal cell 12 and the auxiliary capacitor 14. When the TFT 10 is turned on, the liquid crystal cell 12 and auxiliary capacitor 14 accumulate the voltage of an image signal V.sub.D from the data line 13, thereby maintaining the accumulated voltage until the TFT 10 is turned on again. Because of the parasitic capacitance of the TFT 10, however, a voltage V.sub.S accumulated in the liquid crystal cell 12 and the auxiliary capacitor 14 suddenly changes up to a voltage equal to the voltage in the data line 13 when the TFT 10 is turned on, and thereafter changes to a lower voltage than the voltage in the data line 13 when the TFT 10 is turned off. Meanwhile, the positive polarity voltage and the negative polarity voltage applied to the liquid crystal cell 12 have different absolute values with respect to each other. As a result, when the same picture is displayed on the liquid crystal display panel for a certain time, a DC voltage component is accumulated by a certain amount in the liquid crystal cell 12 according to the progress of frame. This DC component accumulated in the liquid crystal cell 12 causes a residual image to be displayed on the liquid crystal panel when the picture changes.
As described above, a method of changing the common voltage VCOM applied to the liquid crystal cell 12 has been suggested as a strategy for eliminating the residual image caused by the parasitic capacitance of the TFT 10. However, this method fails to adequately compensate for the DC voltage accumulated in the liquid crystal cell 12 because the DC voltage accumulated in the liquid crystal cell 12 changes depending on the voltage on the data line 13. This results in a residual image still appearing in the above common voltage varying method.
Specifically, if a voltage V.sub.D in the data line 13 is 5 V higher than the common voltage V.sub.COM, then a varied voltage .DELTA.V.sub.S(.sub.5V) in the liquid crystal cell 12 becomes large; while if a voltage V.sub.D in the data line 13 is equal to the common voltage V.sub.COM, then a varied voltage .DELTA.V.sub.S(DV) in the liquid crystal cell 12 becomes small. The varied voltage .DELTA.V.sub.S(5V) in the liquid crustal cell 12 when the voltage V.sub.D in the data line 13 is 5 V higher than the common voltage V.sub.COM, and the varied voltage .DELTA.V.sub.5(OV) in the liquid crystal cell 12 when the voltage V.sub.D in the data line 13 is equal to the common voltage V.sub.COM, can be respectively represented by two expressions as follows: EQU .DELTA.V.sub.S(5V) =C.sub.gd (V.sub.gh -V.sub.gl)/(C.sub.LC(ON) +C.sub.5+ C.sub.DS+ C.sub.gd) (1) EQU .DELTA.V.sub.S(OV)= C.sub.gd (V.sub.gh- V.sub.gl)/(C.sub.LC(OFF) + C.sub.st +C.sub.DS +C.sub.gd (2)
where,
C.sub.gd is the capacitance between the gate and drain (or source), PA1 V.sub.gh and V.sub.gl are the high and low voltages applied to the gate, respectively, PA1 C.sub.LC(ON) and C.sub.LC(OFF) are the capacitances of the liquid crystal cell with and without an applied voltage, respectively, PA1 C.sub.st is the storage capacitance, and PA1 C.sub.ds is the capacitance between the source and drain. As seen from the above expressions, the absolute value difference between the positive polarity voltage and the negative polarity voltage applied to the liquid crystal cell 12 increases in accordance with the increase of the voltage on the data line 13, and hence an amount of the DC voltage accumulated in the liquid crystal cell 12 every frame is different. On the other hand, since it is difficult to apply a different common voltage V.sub.COM to each liquid crystal cell 12, the common voltage cannot be changed in response to a voltage variation in the data line 13. For this reason, it is impossible to eliminate a residual image completely in the conventional liquid crystal display device employing the above mentioned common voltage varying method.
For example, it is assumed that the common voltage V.sub.COM was lowered by the intermediate voltage .DELTA.V.sub.S(M) between the varied voltage .DELTA.V.sub.S(5V in the liquid crystal cell 12 when the voltage V.sub.D in the data line 13 is 5 V higher than the common voltage V.sub.COM and the varied voltage .DELTA.V.sub.S(OV) in the liquid crystal cell 12 when the voltage V.sub.D in the data line 13 is equal to the common voltage V.sub.COM, as expressed in the following formula: EQU .DELTA.V.sub.S(M) =[C.sub.gd /(C.sub.LC(ON) +C.sub.st +C.sub.ds +C.sub.gd)-Cgd/(C.sub.LC(OFF) +C.sub.st +C.sub.ds +C.sub.gd).multidot.[(V.sub.gh -V.sub.gl)/2]+[C.sub.gd /(C.sub.LC +C.sub.st +C.sub.ds +C.sub.gd)].multidot.(V.sub.gh -V.sub.gl) (3)
Further, provided that a voltage of 5 V is supplied to the data line 13, a DC voltage of positive polarity (+) accumulates in the liquid crystal cell 12 every frame. Otherwise, provided that a voltage of OV is supplied to the data line 13, a direct-current voltage of negative polarity (-) accumulates in the liquid crystal cells 12 every frame. By this positive or negative polarity DC voltage, a residual image appears on the liquid crystal display panel.
Another alternative for eliminating this residual image is a liquid crystal display apparatus that corrects the image signal every frame, as disclosed in Japanese Patent Lade-open Publication No. Puyng 3-212815, published on Sep. 18, 1991 and filed by Nippon Victor Co., Ltd. The liquid crystal display apparatus according to the Japanese patent publication could eliminate a residual image appearing on the liquid crystal display panel by calculating a varied differential signal on the basis of a differential signal between fields for every frame. That is, the apparatus calculates a differential signal between adjacent scanning lines and the level of the image signal and by then adding the varied differential signal to the image signal. This liquid crystal display apparatus, however, has a disadvantage in that, since it utilizes a differential signal between fields composed of one picture, that is, a differential signal between adjacent scanning lines, the image signal may be distorted. Thus, a distorted picture different from the original picture may be displayed on the liquid crystal display panel.